The present application relates to a quantum cryptography communication apparatus adapted to perform a communication process based on quantum cryptography and also to a communication terminal used in the quantum cryptography communication apparatus. More particularly, the present invention relates to a quantum cryptography communication apparatus having an optical system adapted to transmit and return light, and also having a frequency shifter disposed on a sending side and adapted to shift frequencies of signal light and reference light, wherein the frequency shifter is formed together with a light attenuator for attenuating the intensity level of the signal light in an integral manner by a single acousto-optic device thereby suppressing the adverse effects of backscattered light without causing degradation in performance or reliability of quantum cryptography. The present invention also relates to a communication terminal for use in such a quantum cryptography communication apparatus.
Cryptography is used to transmit information while concealing the information from third parties. Typical cryptographic techniques include public key cryptography using a RSA algorithm, an ElGamal algorithm, etc., and private key cryptography using an AES algorithm, a DES algorithm, etc. The security of the former technique relies on the difficulty in factorization into prime factors and the difficulty in solving discrete logarithm problems. Thus, this cryptography technique is faced with threat of decipherment by quantum computation or other attacks. On the other hand, in the latter technique, a private key needs to be shared by a sender and a receiver in advance. In order to share the private key, the private key is generally exchanged by using the former technique. As with the former cryptography technique, the latter technique is also faced with various attacks, which are making progress day by day. The two cryptography techniques described above rely on the computational security.
Quantum key distribution (QKD) is known as a technique to securely share a private key between two parties. In the QKD, information is carried by a single photon or by weak light obtained by attenuating intensity of laser light. If the information is eavesdropped by a third party, the eavesdropping is detected by the uncertain principle or the no-cloning theorem. This guarantees the security of the QKD.
The QKD can be classified into two types according to the method of detecting weak light. One is single-photon QKD in which a single photon is detected using a single-photon detector realized using an avalanche photodiode (APD) or the like. The other one is continuous variable QKD in which a homodyne detector realized by a photodiode (PD) is used.
Various optical systems for use in the above techniques have been proposed. Of these techniques, much attention has been paid to a plug-and-play technique (see, for example, A. Muller, T. Herzog, B. Huttner, W. Tittel, H. Zbinden, and N. Gisin, Appl. Phys. Lett., 70,793 (1997) and M. Legre, H. Zbinden, and N. Gisin, Quantum Inf. and Comp., 6,326 (2006)). The optical system according to this technique is characterized in that signal light carrying information and reference light for use in detection of the information by interference are passed through the same transmission line twice: once in a forward direction and once in an opposite direction. In this technique, the optical path is selected by using polarization so that the signal light and the reference light travels along the same optical path. This makes it unnecessary to make adjustment of the optical path. Although an optical fiber has large fluctuations in polarization, the fluctuations are cancelled out when light is passed in the forward direction and then in the backward direction. Thus, it is not necessary to made adjustment in terms of polarization.
However, use of the transmission line along which light is passed in the forward direction and the backward direction causes backscattered light generated in the optical fiber to be incident on the detector, and thus the backscattered light functions as noise to the signal light carrying the information (see, for example, D. Subacius, A. Zavriyev, and A. Trifonov, Appl. Phys. Lett., 86, 011103 (2005)).
The APD used in the single-photon QKD reacts to backscattered light even if the backscattered light includes only a single photon. Thus, the backscattered light has a significant adverse influence on detection of the signal light. On the other hand, in the continuous variable QKD, scattered light can interfere with reference light with high intensity compared with the intensity of signal light, and, as a result, noise is generated. To avoid the above problems, it is known to reduce the repletion frequency at which light pulses are emitted from a light source so that backscattered light is not incident on the detector when the signal light is incident on the detector, or the above problems are avoided by transmitting a pulse sequence at equal intervals. However, in the techniques described above, the information transmission rate decreases with the transmission length.
For example, Japanese Unexamined Patent Application Publication No. 2005-268958 discloses a technique in which a frequency shifter (which is a phase modulator driven b, an RF signal with a fixed frequency f1) is disposed on a sending side, and a filter which allows only light with a particular frequency is disposed in front of a photon detector on a receiving side thereby to suppress the influence of backscattered light.
The technique disclosed in Japanese Unexamined Patent Application Publication No. 2005-268958 has the following problems.
1. In quantum cryptography, measurement is performed by interfering signal light having a low intensity with reference light having a high intensity. However, the frequency shifter (light phase modulator) additionally disposed in the optical path causes a reduction in the intensity of the reference light, which results in an increase in excess noise in the measurement performed on the receiving side, and thus a reduction in security or reliability of quantum cryptography.
2. To drive the frequency shifter (light phase modulator), an additional control signal is necessary. This results in an increase in power consumption and an increase in complexity of the apparatus.
3. The frequency shifter (light phase modulator) additionally disposed in the optical path results in an increase in fluctuation of transmittance, which can adversely influence the performance of the quantum cryptography using weak light.